A bird flying level seems to rise as it nears the eye
Rising images in the pupil make the level-flying bird appear to climb
This folio takes the converse of the preceding page: when a bird flies along the line of equality, it appears to rise the nearer it comes to the eye. Leonardo labels the level line g h, the bird moving through g c S, and the eye n, and argues that as the bird's images climb by degrees in the pupil, in every degree of motion, the eye reads it as mounting. Two diagrams accompany the text, an upper construction with the level line and eye, and a lower analogous figure with the eye drawn in a circle sending rays to birds along a horizontal line.
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Approaching level flight reads as rising
When the bird flies along the line of equality, the closer it comes to the eye the more it appears to raise itself. As the bird's images climb by degrees of height in the pupil, in every degree of motion the eye judges the bird to be mounting.
Level line g h, bird through g c S, eye n
Leonardo sets g h as the line of equality, lets the bird move through the points g, c, S along it, and fixes the eye at n. The upper diagram carries these letters and the crossing sightlines that turn a horizontal path into an apparent ascent.
Analogous figure of eye and rising birds
The lower analogous figure repeats the scheme with the eye set inside a small circle and rays fanning up to bird marks on a horizontal line. It shows the same optical rule in a second arrangement.